Electronic imaging sensors usually have an array of m×n photo-sensitive pixels, with x>=1 rows and y>=1 columns. Each pixel of the array can individually be addressed by dedicated readout circuitry for column-wise and row-wise selection. Optionally a block for signal post-processing is integrated on the sensor.
The pixels typically have four basic functions: photo detection, signal processing, information storage, and analog or digital conversion. Each of these functions consumes a certain area on the chip.
A special group of smart pixels, called demodulation pixels, is well-known for the purpose of three dimensional (3D) imaging. Other applications of such demodulation pixels include fluorescence life-time imaging (FLIM). The pixels of these demodulation imaging sensors typically demodulate the incoming light signal by means of synchronous sampling or correlating the signal. Hence, the signal processing function is substituted more specifically by a sampler or a correlator. The output of the sampling or correlation process is a number n of different charge packets or samples (A0, A1, A3 . . . ) for each pixel. Thus, n storage sites are used for the information storage. The typical pixel output in the analog domain is accomplished by standard source follower amplification. However, analog to digital converters could also be integrated at the pixel-level.
The image quality of demodulation sensors is defined by the per-pixel measurement uncertainty. Similar to standard 2D imaging sensors, a larger number of signal carriers improves the signal-to-noise ratio and thus the image quality. For 3D imaging sensors, more signal carriers mean lower distance uncertainty. In general, the distance measurement standard deviation a shows an inverse proportionality either to the signal A or to the square root of the signal, depending whether the photon shot noise is dominant or not.
  σ  ∝      1          A      if photon shot noise is dominant
  σ  ∝      1    A  if other noise sources are dominant